The present invention relates to an exhaust gas purification system and a method for controlling the same. More particularly, the invention relates to an exhaust gas purification system and a method for controlling the same capable of eliminating clogging of a reducing agent injection valve due to solidified urea aqueous solution.
Conventionally, exhaust gas of an internal-combustion engine included in a vehicle contains nitrogen oxides (hereinafter referred to as “NOx”) and particulate matter (hereinafter referred to as “PM”). Among them, NOx can be reduced to purify exhaust gas using, for example, a urea SCR system. The urea SCR system includes: a reducing agent supply apparatus for pumping urea aqueous solution as a reducing agent from a storage tank by a pump and supplying the urea aqueous solution from a reducing agent injection valve into an exhaust pipe; and SCR catalyst, a type of exhaust gas purification catalyst capable of adsorbing ammonia. The urea SCR system causes ammonia produced by decomposition of urea aqueous solution to be adsorbed to the SCR catalyst and causes NOx contained in exhaust gas to be reacted with the ammonia in the SCR catalyst, thereby purifying the exhaust gas.
On the other hand, PM can be collected to purify exhaust gas using, for example, a diesel particulate filter (hereinafter referred to as “DPF”). The DPF is placed in an exhaust pipe of an internal-combustion engine and, when exhaust gas passes through the DPF, the DPF collects PM contained in the exhaust gas. The exhaust gas purification system including the DPF performs forced regeneration control at appropriate times in order to prevent clogging of the DPF, in which the temperature of the DPF is increased to about 500-600° C. to force PM depositing on the DPF to be burnt. Recently, regulations for purifying exhaust gas has been tightened. With this trend, an exhaust gas purification systems including both the DPF and SCR catalyst has been increasingly used.
By the way, in general, the urea SCR system, when the internal-combustion engine is stopped, collects urea aqueous solution left in a reducing agent supply passage (e.g., see JP-A-2009-215891). This can prevent urea aqueous solution left in the reducing agent supply passage from getting frozen to cause clogging of the reducing agent supply passage.